Damping devices of this type are intended to dissipate under control rapid fluctuations in the stresses occurring which may possibly be detrimental to the relevant machines and/or tools used on them, thereby to damp and reduce the noise. The noise is caused by vibrations in the machine which can be propagated in or from the latter as body sound and which by vibrations of the surfaces of the machine, its parts and/or the tools likewise set the air surrounding it into vibrations in the audio frequency range of approximately 16 to 16,000 Hz and generate air-conducted noise.
This phenomenon appears particularly in the case of abrupt operations, of which the creation of noise during cutting by shears and punches may be regarded as typical. Three consecutive phases can be distinguished in such shears and punches, namely after the application of a restrainer or application of an offcut holder--i.e., after the workpiece to be machined has been anchored--a first phase of the elastic stressing, in which the stress occurring in the material to be cut does not yet exceed the yield point. When the yield point is exceeded the actual cutting operation commences as a second plastic phase. The cutting edges then penetrate into the workpiece up to 0.2 to 0.5 times its thickness, depending upon the strength of the material to be cut. The stress in the workpiece increases up to its ultimate strength. In the third phase of cutting, cracks appear, commencing at the cutting edges, and are quickly propagated into the interior of the workpiece and ultimately lead to the fracture of the workpiece at the point of cut. The machine or the machine frame is now subjected to force and resiliently prestressed in accordance with its spring characteristic until the maximum cutting force is attained. At the moment of parting of the workpiece, the machine or the machine frame is abruptly relieved. This gives rise to more or less strongly pronounced vibrations in the direction of all degrees of freedom. This phase is therefore also called the vibration phase. This relief is generally completed in the very short time e.g., approximately 0.1 ms. Due to the abrupt short-term transition of forces, vibrations are excited in the machine which are propagated as body sound to the surface of the machine or of the machine frame and are radiated from there as air-conducted sound. The height of the noise level is then a function of the magnitude of the force transition or of the value of the cutting force and of the time in which the relief is completed. This phenomenon could only be totally prevented if it were possible to eliminate the force transition, for which purpose the force build-up phase and the force dissipation phase would have to follow a substantially similar pattern.
A number of measures and apparatuses have already been proposed in order to solve this problem, such as a roof-shaped construction of the ram in punching. Another possibility, also suitable for shears, lies in the use of so-called cutting shock dampers. By this means, although the cutting requirement is not reduced, it is achieved that after the workpiece has been parted the machine is not relaxed in an extremely short time, whereby the generation of vibrations is largely prevented. The mode of operation is that the moving tool or the machine ram or the shear blade strikes against a more or less hard stop at the moment when the machine frame is most highly stressed, and is thus impinged by a counterforce, whereby a full vibration of the machine frame in the relief phase is prevented. However, such damping devices necessitate an optimally accurate co-ordination between their rigidity and that of the machine, so that universal application is difficult. Although such damping devices leave the cutting or working force substantially unchanged, nevertheless the force decay phase is considerably prolonged in time and the force transition substantially reduced.
A damping device of the type initially defined is known (German Offenlegungsschrift No. 25 12 822 to Horst Schenk and published Sept. 30, 1976) and substantially comprises a damping cylinder, a damping piston reciprocatingly movable in its damping cylinder, which co-operates with the hydraulic pressurised medium contained in the damping cylinder between the damping piston and the base, and an attached pressure accumulator. This results in the following mode of operation: before the actual parting of the workpiece, the moving tool or the moving machine part alights upon the damping piston of at least one damping device (depending upon whether one or more damping devices are used). During the subsequent further descent of the moving tool the pressurised medium is displaced out of the damping cylinder through a line with throttle into the pressure accumulator. By this means the force required to intercept the moving tool is built up slowly. At the moment of parting the damping device absorbs the entire pressure force and the subsequent movement of the moving machine part is intercepted by means of the line with throttle until a relief line is opened and can thus become operative. The residual movement of the moving tool then occurs with greater velocity, because the pressurised medium can escape out of the damping cylinder through the relief line without much resistance into the pressure accumulator. After completion of the work movement and during the ascent of the moving tool, in the known damping device the damping piston returns into the initial position. Simultaneously the damping cylinder is filled afresh with pressurised medium from the pressure accumulator.
The essential point is the detection of the moment of separation, i.e., the moment of maximum loading. For this purpose, in the known damping device the elongation of the machine frame dictated by the loading is detected, as a function of which the flow cross-section of the line is modified, so that it is the smallest at the moment of maximum loading, whilst after separation the flow cross-section increases slowly back to its initial value. For this purpose in the known damping device a transmission linkage articulated to the machine frame is provided, which transmits the elongation of the machine frame occurring during the machining operation before the tool breaks through the workpiece to a relief valve arrangement. The latter has, arranged in a control cylinder chamber, the housing of which is mounted firmly on the damping cylinder, a control piston movable axially by the transmission linkage, the linkage end of which is frustoconical in shape and surrounded by an associated substantially cylindrical wall. The pressurised medium displaced by the damping piston is fed to the chamber formed at this end and discharged through a line in the control housing at the other end of the control piston into the pressurised medium accumulator. The elongation dictated by the loading causes the control piston, through the intermediary of the transmission linkage, to be moved in the control housing, so that the flow cross-section is gradually reduced. The stroke length of the damping piston is preadjusted, so that at the moment of maximum loading the flow cross-section between control piston and control housing is practically closed. For reasons of convenience a shunt throttle may also be provided, by which a minimum passage quantity is ensured.
The known damping device is relatively complicated, whilst a difficult adjustment of the transmission members is necessary due to the comparatively small absolute elongations of the machine frame. Furthermore, due to the direct intervention with the machine frame, the subsequent equipment of existing machine tools is only possible with great difficulty and great expense. This is particularly disadvantageous when the behaviour of the workpiece during machining changes frequently--e.g., because of different material or different dimensions. Furthermore the line with throttle is arranged asymmetrically, so that particularly at the maximum pressure build-up, staggered attacking forces come into effect, by which the desired effect is detrimentally influenced and above-average wear on one side is caused.
A damping device is also known (German Patenschrift, No. 26 53 714 to Adam Dittner and patented on Nov. 9, 1978.), wherein these complicated measures are avoided. In this known damping device the pressure in the damping cylinder is detected, converted into an electrical signal and compared with an adjustable desired value. Slow variations in the pressure and hence in the signal are compensated by means of an electrical integration member with an appropriate time constant. A rapid variation in the pressure, as when the tool breaks through the workpiece, then produces an output signal which is compensated, not immediately but after a time delay, by the integration member. The time delay permits a damping. The pressure in the damping cylinder chamber which counteracts the movement of the moving machine part, is maintained at the desired value by means of a proportional regulator valve adjustable by the integration member. However, the abruptly rising pressure occurring at the breakthrough is followed by the opening of the proportional regulator valve not immediately, but only with time delay in accordance with an e-function, whereby the damping is achieved.
In a mechanical solution, instead of the electrical integration member a flow-sensitive valve is used (German Offenlegungsschrift No. 28 04 185 to Louis Francescon Canieri and published on Aug. 10, 1978) which terminates the flow in the case of a sudden pressure rise. Both damping devices are essentially independent of the machine tool and connected to the latter by pressurised medium lines. Space is therefore required outside the machine tool. In addition, these damping devices are only satisfactorily applicable to hard presses or the like. Furthermore the effect of the damping device necessarily occurs only after the breakthrough. An adjustment to a point in time, e.g., just before the moment of maximum loading, is impossible. The effects of the abruptly occurring shock cannot therefore be totally eliminated.